Fertilization: Part 2:
Prevention of Polyspermy
Gilbert - Chapter 7
pp. 187-192
Today’s Objectives
• Identify the following important components
of the process of fertilization: gametes,
spermatogonia, acrosome, flagellum, tubulin,
oocyte, pronuclei, vitelline membrane, zona
pellucida, resting membrane potential,
capacitation, cortical granule reaction
• Identify the structure of an oocyte
• Recognize the harm of polyspermy
– Describe various ways that polyspermy is inhibited
• Recognize that fertilization is species-specific
Fertilization: 4 major events
• Sperm and egg make contact and must
recognize each other as the same
species
• ONE (and only one) sperm enters egg
• Fusion of the genetic material
• Activation of egg to begin development
Polyspermy
• Monospermy is the norm
– Restores the diploid chromosome number
– Sperm centriole becomes the mitotic
spindle
• Polyspermy is disastrous
– Results in triploid nucleus
– Multiple mitotic spindles form
Dispermic
Sea Urchin
egg
Dispermic Human Egg Note 4 centrioles
Prevention of Polyspermy
• Usually done by preventing multiple
sperm from entering the egg
• Sea Urchins have 2 mechanisms:
– Fast Block
• Involves a change in egg cell membrane
potential
– Slow Block
• Involves exocytosis of the cortical granules in
the egg
Fast Block to Polyspermy
(Sea Urchin model)
• The egg has a different ionic concentration
from the seawater in which it exists
• Egg has lower sodium ion concentration;
higher potassium concentration
• This is maintained by sodium/potassium
pumps in the egg cell membrane
• The difference in charge across the egg
membrane can be measured as -70mV and is
called the resting membrane potential
• Sodium-Potassium Pump
– Pumps Sodium Out of cell
– Pumps Potassium Into Cell
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Fast Block to Polyspermy
(Sea Urchin model)
• 1-3 seconds after first sperm binds, the
membrane potential shifts to +20 mV
• Sperm can no longer fuse to the egg
• Experimental evidence - Polyspermy can
occur if*:
– Eggs are supplied with an electrical current
that keeps charge at -70mV
– Fertilization occurs in water with a low
sodium ion concentration
*Jaffe 1976
Slow Block to Polyspermy Sea urchin model
• Slower, mechanical, permanent block
• Occurs about a minute after sperm-egg
fusion
• Upon sperm entry cortical granules fuse
with the cell membrane and release
several molecules
Cortical Granule Molecues
• Cortical granule serine protease
– Releases vitelline membrane from its anchors to the cell
membrane
– Clips off bindin molecules
•
Mucopolysaccharides
– Cause osmotic gradient
– Water rushes into space between vitelline envelope
– Vitelline envelope expands (lifts) and becomes the
fertilization envelope
• Peroxidase
– Hardens the fertilization envelope
• Hyaline
– Forms a coating around the egg, protects during early
embryonic development
Formation of fertilization envelope in sea urchin
Mammalian Cortical Granule
Reaction
• Does not form a fertilization envelope
• Does modify Zona Pellucida so sperm
cannot bind
• In mice, cortical granules cleave an
essential portion of the ZP3 molecule
Calcium and the Cortical Granule
Reaction
• Upon fertilization, intracellular concentration
of Calcium ion in the egg increases
• This is necessary for the fusion of cortical
granules with the cell membrane
• Calcium comes not from outside the egg, but
from inside the egg itself
• The fusion begins near the site of sperm
entry and continues in a wave across the egg
• A similar wave of calcium ion release can be
observed
Calcium experiments - Ca2+ is
directly responsible for cortical
reaction
• A23187 is a calcium ionophore
– Transports Ca2+ across lipid membranes
• Placing sea urchin embryos in sea
water containing A23187 results in
cortical granule reaction & fertilization
envelope to rise (without presence of
sperm)
• If Ca2+ chelator is injected into egg, no
cortical reaction occurs
Fertilization: 4 major events
• Sperm and egg make contact and must
recognize each other as the same
species
• ONE (and only one) sperm enters egg
• Fusion of the genetic material
• Activation of egg to begin development